You don't need software to clean up your pc. You just need to go to the control panel and uninstall anything you don't absolutely need.
You should also run "msconfig" and take all the extra crap out of startup and services that you don't absolutely need.
If you want a couple of free Anti-virus/spyware blocking programs I would suggest:
Spybot Search & Destroy from downloads.com
Run one of those prior to cleaning up your pc to remove any malware/spyware/viruses.
If you are not sure what to get rid of, just do a search on the items you think you may not need. Google/Yahoo are very good at telling you what it is/does.
Lastly when you go to the services tab on the system utility, check the box that says "hide all microsoft services." What's left is the optional stuff. On the startup tab, you can usually get rid of everything. If something is broken, put it back. That will seriously speed up your computer.
Using task manager to see what is running is also a good way to decide what is slowing down your computer.
AVG's pc tuneup 2011 isn't free, but it is reasonable and does a pretty good job. You might want to spring for a few bucks if you don't want to do the extra work. Good luck.